Conventional polymer nanocomposites consist of: (1) inorganic nanofiller as a property/performance booster; (2) a polymeric matrix providing processability and holding the reinforcements together into a solid; and (3) surface ligands to control nanofiller dispersion, essential to delivering the promised performance enhancement (see Scheme 1 (FIG. 11)).1-3 To obtain greater property reinforcement, a large volume fraction of nanofiller (νfiller) is often desired, especially for optical nanocomposites.4, 5 However, the probability for macro-phase separation tends to be larger at higher volume fraction. To maximize nanoparticle loading while maintaining uniform particle dispersion, νmatrix should be zero.
To achieve the maximum optoelectronic property enhancement of polymer nanocomposites, very high nanofiller loading fractions is often desired. For traditional polymer nanocomposites, where inorganic nanofillers with superior optoelectronic properties (e.g. high refractive index) are strongly incompatible with polymeric matrices, obtaining well-controlled nanofiller dispersion at high loading fractions is very challenging, which compromises the promised property enhancement of the nanocomposites. Current technology solutions involve the use of capping agents to stabilize nanofiller dispersion and suppress macroscopic phase separation between capped nanofiller and matrix polymer, which, however, take up significant volume fractions. It is even more challenging when precise control of nanofiller distribution or concentration gradient is required and/or multiple functionalities need to be incorporated into the nanocomposites.
An analogous matrix-free system is the so-called organic/inorganic “solvent-free nanofluid”, whose fluidity is suited for applications in heat-transfer fluids, lubricants, and liquid electrolytes.6-10 Solid-state polymer nanocomposites, on the other hand, require a higher level of structural integrity. Tchoul et al. first demonstrated a mechanically robust matrix-free assembly of inorganic nanoparticles surface grafted with thermoplastic polymer brushes.11 Polymer brush chain interpenetration and entanglement, which occurs only above a critical molecular weight for entanglement,12 serves as physical cross-links to ensure good mechanical properties.13 
Alternatively, in the absence of entanglements, mechanical integrity can be provided by chemical cross-linking, which is especially important for thermoset polymer nanocomposites with a Tg below room temperature. A low Tg brush can be useful for promoting flow and moldability, while setting the meso- or macroscopic assemblies of grafted nanoparticles into desired architectures can be enabled by cross-linkable brush polymers. Using two kinds of complementary reactive polymer brush grafted SiO2 nanoparticles, Dach et al. synthesized chemically cross-linked “matrix-free” nanocomposites.14 However, the reported shear moduli of the cross-linked nanocomposites were low, due to incomplete network formation between the two immiscible polymer brushes. A suggested solution is to use the same crosslinkable moiety for all the SiO2 particles. The van der Waals (vdW) core-core attraction between high refractive index metal oxide (e.g. TiO2 or ZrO2) nanoparticles within a polydimethylsiloxane (PDMS) matrix is often much higher than the thermal fluctuations at room temperature (5 to 10 kBT), which encourages particle core —core agglomeration.15-18 Intuition suggests that a densely grafted long chain polymer brush would screen this core-core attraction and also enable entanglement. However, achieving both high graft density and high molecular weight of a polymer brush is not only experimentally challenging but reduces the achievable νfiller.
The present invention is directed to overcoming these and other deficiencies in the art.